1. Field of the Invention
The present invention relates generally to optical waveguides and, more particularly, to fluid light guides for the transmission of a wide range of wavelengths. Specifically, the present invention relates to liquid/gel light guides having a high critical angle of acceptance for illumination purposes and a continuous process for the manufacture thereof.
2. Description of the Prior Art
Numerous kinds of waveguides or optical fiber structures have been developed for the transmission of electromagnetic radiation of various wavelengths between infrared to ultraviolet, and in particular in the visible light region. These fiber-optic devices are utilized in numerous industries, in particular the medical, communication and lighting industries, for a number of different purposes. In many instances, it is desired to transmit visible light along a substantial distance to locations which are physically remote or very difficult to reach.
One popular type of light waveguide which is known in the art employs the principle of total internal reflection to transmit light along a column or core of material, usually quartz or glass. For purposes of explanation, a light guide is the optical equivalent of a water pipe. It generally consists of a low refractive index cladding, which is the equivalent of the pipe in this analogy, and a high refractive index core through which light is channeled by total internal reflection. The most widely known and used light guide is glass fiber. In this instance, the outer layer of glass is made of a lower refractive index material than the core glass. Other examples of fibers used in waveguides include fused silica, acrylics and other plastic materials, polymers and liquid or gel materials.
In a typical glass or quartz light guide, the fibers along the core generally consist of a central core of glass or quartz surrounded by an optical cladding layer which has a refractive index lower than that of the core material. Light incident on the junction or interface between the core and cladding will generally be totally reflected if the angle of incidence of the light, measured from a line radially normal to the junction or core surface, is greater than a certain specified critical value. Thus, light can be transmitted along the fiber and around bends, provided that the radius of the bent waveguide is not too small so as to maintain the proper angle of incidence. U.S. Pat. No. 5,684,907 discloses one particular type of glass waveguide core which is coated by an aerogel cladding layer.
Fiber optic devices utilizing glass cores transmit wavelengths primarily in the visible light range. Quartz, on the other hand, transmits a wide range of wavelengths from the ultraviolet to the near infrared. Unfortunately, quartz waveguides are very fragile as well as being very expensive and difficult to produce. Although ideal for communications purposes, which require the transmission of low energy radiation over long distances, quartz fibers present great difficulties for the transmission of relatively large amounts of power over shorter distances. Moreover, a principal problem with these fibers is that they must be very thin to provide required flexibility, which is a significant disadvantage when the fibers are fragile as in quartz. In addition, the power densities created in thin fibers with laser applications can be high enough to cause breakdown of the core material. In addition, it is very difficult to successfully launch broad light beams that may be 50 mm or more in diameter, or large diameter laser beams, into fibers of this type without high losses.
Some optical fibers of this type have been formed from finely drawn quartz capillary tubes filled with a liquid, but the same problems of fragility, alignment, and breakdown of the material generally prevent their use in any of the aforementioned applications requiring high power levels. Bonding large numbers of single glass fibers together to form flexible fiber-optic bundles of much greater diameter than single fibers has been useful for a number of applications. However, with use the individual fibers begin to fracture, leading to increasing numbers of dead spots and decreasing efficiency of the bundle. In addition, because only the cores of individual fibers transmit light, and since these make up only a percentage of the cross-sectional area of a fiber-optic bundle, light falling on the material between the fibers is not transmitted, but is rather absorbed. This results in initial losses proportional to the non-transmitting area of the bundle, and if the energy of the incident light beam is high enough, destruction of the bundle due to energy absorption. Thus, waveguides of this type cannot usually be used in the transmission of high energy radiation.
Optical fibers with plastic cores have been previously described. However, because of the nature of the materials from which they are constructed, they tend to have high light losses, can generally operate only at low temperatures and are generally suitable only for non-critical, low light applications. In addition, aerogel materials have been noted for their low refractive index as illustrated in U.S. Pat. No. 5,496,527 and in the previously referenced U.S. patent.
Liquid and gel-core light guides, such as disclosed in U.S. Pat. Nos. 4,045,119 and 5,452,395, offer the advantage that a very large core fiber can be constructed. High light throughput is possible in such light guides without the dead space between adjacent circular fibers associated with fiber bundles. The main problem with these large diameter, liquid core fibers is the availability of low refractive index cladding and high refractive index, low loss core materials. Put another way, the problem is to find workable combinations of solid cladding and fluid core materials such that the refractive index differential provides an acceptable numerical aperture for the resultant fiber. The numerical aperture is a measure of the acceptance cone of light that can be coupled into the fiber, and the objective is to achieve as high a numerical aperture as possible.
Currently, liquid light guides are typically made with a calcium chloride core and fluorinated ethylene polymer (FEP) cladding. These materials are relatively expensive and require exacting manufacturing techniques and standards. Other core materials such as one of the silicones or polymer material can be used. Similarly, lower cost claddings may be used, either as a simple tube, or as a plastic tube coated with a low refractive index layer on the inside surface of the tube. However, the materials currently available are expensive or suffer from limited light transmission or wavelength range performance. Moreover, the coating procedures during manufacturing are batch processes which are very cumbersome for making such waveguides in substantial lengths such as 50-100 feet or more.
U.S. Pat. Nos. 5,684,907, 5,692,088 and 5,790,742 all disclose optical fiber structures and arrangements. However, the '742 patent is typical in that it requires supercritical processing to prepare aerogel films. This would be very difficult to implement commercially in that the process must somehow coat the fiber, maintain it in a wet state, transfer it to an autoclave and then supercritically extract it. Normally aerogels are in fact defined by the need for supercritical drying, while xerogels refer to porous solids created by normal evaporative drying of a wet gel (alcogel). The present invention, however, prepares aerogel-like, hydrophobic films with low refractive index values without supercritical drying, making the materials aerogel-like xerogels better described as hydrophobic ultraporous xerogels, non-supercritical aerogels, or aerogel-like materials/films. Thus, the aerogel-like thin film material of the present invention as described below are not aerogels in the classical sense, ther4by distinguishing them from the above '907, '088 and '742 patents.
Therefore, there is still a need for a very low refractive index coating that could be applied to the inner surface of a low cost plastic tube such as polyethylene or the like to form the basis for a fluid light guide wherein water or other superior transmission core liquids or gels could be used. Moreover, the coating material must be unreactive with the fluid core and readily applied to long tubular interior surfaces in a continuous process to provide substantial economic benefits. The result would be a low cost light guide with superior wavelength and transmission performance.